An example of known vehicle drive systems using laterally positioned power units such as internal combustion engines is disclosed in Japanese Preliminary Publication of Utility Model No. 55-83119. The four-wheel drive system therein taught uses a front-wheel drive differential gear assembly driven by a drive gear held in mesh with an output gear of the power transmission gear unit and with a drive gear for a rear-wheel drive differential gear assembly. The front-wheel differential gear assembly is, thus, operative to drive the drive axles for the front road wheels and to transmit driving power to the rear-wheel differential gear assembly. By reason of the particular arrangement of such a drive system, a low-and-high speed shifting mechanism provided in a prior-art four-wheel drive system of the above described character is required to be incorporated in the gear train included in the power transmission gear unit. Unless the type and design of the power transmission gear unit to be put to use for the four-wheel drive system is equipped with such a low-and-high speed shifting mechanism in the gear train, it is impossible to make a low-and-high speed shifting mechanism to be incorporated in the prior-art four-wheel drive system. The present invention contemplates elimination of these and other problems which have thus far been inherent in prior-art four-wheel drive systems for wheeled vehicles.